Many people think that they have eat enough food to support their body but is not ture because some food that they eat cannot help their health, moreover it can demage their health also.

Dieting is the practice of ingesting food in a regulated fashion to achieve a particular objective. In many cases the goal is weight loss, but some athletes aspire to gain weight (usually in the form of muscle) and diets can also be used to maintain a stable body weight.

Energy obtained from food

The energy humans get from food is limited by the efficiency of digestion and the efficiency of utilization. The efficiency of digestion is largely dependent on the type of food being eaten. Poorly chewed seeds are poorly digested. Refined sugars and fats are absorbed almost completely. Despite the claims of certain popular diets, chewing and digesting does not use a substantial amount of the energy offered by any food (that anyone would want to eat). Even celery, known for being low in caloric value, contains enough sugars (including sucrose, glucose and galactose) to easily compensate for the cost of (energy invested in) chewing it. (See Food & Nutrition Information Center.)The efficiency of energy utilization by skeletal muscles is around 20 percent. That is, of the chemical energy used, 20 percent does work and 80 percent creates heat.

Proper nutrition

Humans require essential nutrients from six broad classes: proteins, fats, carbohydrates, vitamins, minerals, and water. Essential amino acids (protein) are required for cell, especially muscle, construction. Essential fatty acids are required for brain and cell wall construction. Vitamins and minerals are essential for many functions.Any diet that fails to meet minimum nutritional requirements can threaten general health (and physical fitness in particular). If a person is not well enough to be active, weight loss and good quality of life will be unlikely.The National Academy of Sciences and the World Health Organization publish guidelines for dietary intakes of all known essential nutrients.Sometimes dieters will ingest excessive amounts of vitamin and mineral supplements. While this is usually harmless, some nutrients are dangerous. Men (and women who don’t menstruate) need to be wary of iron poisoning. Retinol (oil-soluble vitamin A) is toxic in large doses. As a general rule, most people can get the nutrition they need from foods (there are specific exceptions; vegans often need to supplement vitamin B-12). In any event, a multivitamin taken once a day will suffice for the majority of the population.A sensible weight-loss diet is a normal balanced diet; it just comes with smaller portions and perhaps some substitutions (e.g. low-fat milk, or less salad dressing). Extreme diets may lead to malnutrition, and are less likely to be effective at long-term weight loss in any event.

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